For several years, research has been conducted in attempts to establish communication through living neurons, to convey to the human brain information which can no longer be provided by a person's own eyes or ears, to stimulate paralyzed muscles, to stimulate autonomic nerves, to control bladder function or pace the heart, or to control prosthetic limbs.
It is well known that electrical stimulation of certain nerves and certain regions of the brain can be perceived consciously, and research is being performed on methods of stimulating nerves in ways that can provide useful information to a person whose ability to hear or to see has been lost.
To replace normal sensory and motor function with a neural prosthesis, electrical communications must be made between the prosthesis and living neurons. Such connections must be made by extremely small electrodes, in order to isolate currents within small regions of living tissue. Active electrode sites can be placed very close to nerve cells, and electrical activity at the active electrode sites can be used to provide stimulation to the nerves. To limit the mechanical trauma caused by insertion and chronic presence of electrode structures, the entire electrode structure and associated conduction wires must be as small as possible consistent with the required ability to conduct electrical energy, and must be made of materials which will not react with the living body.
Implanted electrodes and the conduction wires connected to them must be electrically insulated very effectively, because of the very small voltages and currents being utilized. The localized nature of the electrical potential gradient which must be detected by a microelectrode, and the fragility of neurons, dictate a microelectrode tip with small dimensions.
Further, present neuro-stimulation devices require a large number of electrodes placed in close proximity to neural structure to facilitate effective stimulation. In addition, the neuro-stimulation devices require a hermetic electronic housing where the stimulation signals and power are generated. Because the housing is large compared to the stimulation electrodes, the package may need to be surgically placed in a location remote from the stimulation site.
It is therefore required that there be a conductor cable connecting the housing to the electrodes. With the requirement forever increasing numbers of electrodes, conduction wires with ever-increasing numbers of individual channels are needed, and thus ever increasing numbers of conduction pathways.
Because the conduction wires are located in the body, they must be made to withstand billions of micromovements to facilitate continuous operation over the long-term.
Also, conduction wires and electrodes must be constructed of bioresistive, biocompatible materials that do not cause adverse tissue reactions and that allow the structure to endure and function within the hostile electrolytic environment of the human body.
The structure of neuro-stimulation devices should also be reliably producible and relatively inexpensive to fabricate.
Implantable neuro-stimulation devices should:                (a) be small in cross-section or small in overall size.        (b) be fabricated from bioresistive materials.        (c) be extremely resistant and robust under billions of micromovements of the implantee.        (d) be supple and flexible (be able to withstand significant strains).        (e) be able to support a large number of conductors.        (f) maintain a reliable electrical connection between electrode and housing.        (g) be manufacturable using reliable/economical methods of production.        
To meet these demanding, requirements, a photolithographic method of fabrication has been developed.
Platinum electrodes and conduction wires can be conveniently formed using standard techniques such as laser cutting of platinum foil, chemical etching of platinum foil (see for example, R. P. Frankenthal, et. al., Journal of Electrochemical Society, 703(123), 1976).
Alternatively, a well-known photolithographic method whereby a thin coating of platinum is vacuum deposited or sputtered through a photomask, with subsequent electroplating to increase the thickness of the platinum can be used. For example, M. Sonn, et al., (Medical and Biological Engineering, pp. 778–790, November 1974) and M. Sonn (A Raytheon Company Publication PB-219 466, available from the U.S. National Information Service, U.S. Department of Commerce) used, amongst other substrates, the polyfluorocarbon FEP as a substrate onto which platinum conductors and electrodes were sputtered, with the electrode and conductor patterns defined by photolithographic etching means.
G. M. Clark, et al., (Journal of Laryngology and Otology, Vol. XC/No. 7, p623–627, 1976) describe a multi-electrode ribbon-array using a thin 0.1 μm layer of RF sputtered platinum onto FEP, subsequently insulated with FEP, and the electrode stimulating areas exposed. An array of platinum can be made to adhere to an FEP substrate insulated with additional FEP, and exposed at electrode stimulating areas. Bending tests on the array indicate that it is both flexible and strong.
H. D. Mercer, et al. (IEEE Transactions on Biomedical Engineering, Vol. BME-25, No. 6, November 1978) describes a planar lithographic technique for fabrication of a microelectrode array for a cochlear prosthesis using a sputtered platinum layer with thin molybdenum and tungsten substrates.
G. A. May, et al. (IEEE Transactions on Electron Devices, Vol. ED-26, No. 12, December 1979) describe an eight-channel tantalum-on-sapphire multielectrode array design using planar photolithography. The sapphire substrate was chosen for its electrical and mechanical properties, tantalum was applied as the conductor metal, and platinum was applied as the stimulation electrode material.
C. R. Pon, et al. (Ann. Otol. Rhinol. Largngol. 98(6) 66–71, 1989) attempted to form a standard “ring type design” electrode array by using planar photolithography to define the electrode features, RF sputtering platinum onto a polyimide substrate, rolling up the film substrate into a cylindrical shape, and filling it with medical grade silicone rubber.
J. L. Parker et al., in U.S. Pat. No. 5,720,099, describe a photolithographic technique for fabricating an elongated electrode array assembly by first depositing pads on a sacrificial layer, adding wires to the pads (such that the wires are self-supporting when the photoresist mask is removed), then embedding the wires and pads in an insulating material such as silicone elastomer, and finally removing the sacrificial layer. Importantly, a photolithographic process is used to produce the electrode assembly using a sacrificial layer as the initial base.
Those familiar with the art of photolithography and electrochemical deposition processes used in the microelectronics industry will appreciate that there are a number of well established technologies for forming micro patterns of metals and polymer encapsulation thereof.
From commonly owned which is hereby incorporated by reference, there is known an implantable electrode array which is incorporated into a neuro-stimulation device such as cochlear implant.
To better understand and appreciate the present invention, it will be helpful to briefly review an existing implantable medical assembly that is representative of other tissue-stimulating systems. An implantable medical assembly of the type currently fabricated is described in U.S. Pat. No. 6,374,143 B1.
As described in U.S. Pat. No 6,374,143 B1, and as illustrated in FIGS. 1 to 4, such existing implantable medical assembly is explained.
FIG. 1 is an implantable medical assembly having biologically compatible film within which electrodes and conduction wires are connected to the electrode to provide the stimulation signal for human nerves according to prior art. A polymer film 10 has three electrodes (1, 2 and 3) and one conduction wire 8 per electrode, disposed therein. The electrodes 1, 2, and 3 and conduction wires 8 can be fabricated from a biologically compatible and inert metal such as platinum, tantalum, rhodium, rhenium, iridium or alloys thereof, or a combination of two or more alloys and/or metal layers thereof.
The electrodes 1, 2, and 3 and the conduction wires 8 are held in place by an inert film material 10, preferentially the polyfluorocarbon FEP, although any biologically inert, high dielectric constant flexible material may be suitable. As shown in FIG. 1, each conduction wire 8 is connected to each electrode to provide a signal from the stimulator to the human nerves. Those skilled in the art will note that a myriad of possible configurations for the electrodes are possible according to neural shapes, sizes and positions.
The conduction wires 8 have an approximate width of 10–100 μm and an approximate thickness of 2–50 μm. The thickness of the encapsulating film 10 is about 20–100 μm.
Furthermore, numerous studies have been conducted to identify the biocompatibility of various implant materials (see for example “Biocompatibility of Clinical Implant Materials”. Volumes 1 and 2, edited by David F. Williams, published by CRC Press, Inc., Boca Raton, Fla., USA). Some commonly used biomaterials, well known to those skilled in the art, include titanium (and some alloys thereof), platinum, tantalum, niobium, iridium, gold, some ceramics (such as alumina), certain carbon materials, some silicones, and polymers such as the fluorocarbons FEP, PTFE, PVDF, PFA, PCTFE, ECTFE, ETFE and MFA (a copolymer of TFE and PVE), polyethylene's, polypropylenes, polyamides and polyimides.
FIG. 2 is a cross-sectional view of section ‘A—A’ of FIG. 1 showing some embedded metal electrodes and conduction wires. The electrode 1 is exposed to the human nerves to transfer the stimulation signals from the conductor 8.
FIG. 3 is a planar view to illustrate where to fold-in and fold-out an implantable medical assembly according to prior art. FIG. 4 is a perspective view showing the film being folded over along the folding-in and folding-out lines L1, L2 and L3. To make a suitable shape and size for a neural stimulation implant assembly 10 such as a cochlear implant, the implantable medical assembly needs to be folded along the virtual in-folding and out-folding lines L1, L2 and L3 established by the manufacturer.
These lines L1, L2 and L3 are not actually marked on the film 10 in the prior art. When folding the medical assembly, careful handling of the assembly is required. For example, one stimulation implant may need to incorporate multiple folds or more without impairing the structure of the implantable medical assembly.
In prior art shown from FIG. 1 to FIG. 4, the conduction wires 8 having a straight shape are easily fractured because of the continuous movement of the tissues of the body following implantation. This can cause severe problems for the implantee.
The implantable medical assembly requires discrete electrical continuity of the individual conduction wires. Also, electrodes should be maintained to ensure proper signal transfer between target nerves and the implant housing wherein electronic circuits to control the nerves reside. If only one conduction wire is fractured, partial or total malfunction of the implant may result.